1. Field of the Invention
The invention relates to superficial molding on glass sheets, particularly glass sheets for windows of automobiles, yielding multifunctional units to be mounted in the automobile body.
2. Description of the Prior Art
Superficial molding on a glass sheet consists of adding one or more functional elements to at least a segment of the periphery of the sheet, to yield a multifunctional unit to be mounted in the automobile body. This technique offers major advantages and opens the door to numerous applications. In the case of a windshield, a rubber seal may be molded over the entire periphery of the glass, enabling the windshield to be mounted directly in the window aperture of the body by adhesive bonding. In this way the window can be mounted flush with the body, with major improvement in the air drag properties of the vehicle. In the case of a side window, one may carry out superficial molding on a door post or an entire door, to yield a unit with the glass already mounted, which unit can then be directly installed on the rest of the body, on the assembly line. Also, by superficial molding a hatch or rear door may be produced which includes the rear window. An advantage of all these possible applications of superficial molding to glass employed in automobile manufacture is that a number of individual assemblyline operations may be dispensed with, with a consequent reduction in manufacturing time.
Tempered glass is particularly suited as a substrate for superficial molding, due to its excellent resistance to both mechanical and thermal shock, but the superficial molding technique may also be used with other rigid transparent materials, such as, for example, ordinary glass or laminated glass.
In general, superficial molding on an aribitrary substrate is carried out by injection molding a plastic material over at least part of the periphery of the substrate object which the latter is held pressed between two mold plates by appropriate clamping means. Prior to this injection, a central evacuated zone is produced, with the aid of a hermetically sealed toric seal disposed in the mold, said evacuated zone aiding in the holding of the substrate object. The molded region is delimited by rigid bosses or a series of metal prominences, which may be coated with other materials and which can be provided in the structure of the mold. Classical molding methods employ high injection pressures, necessitating good mechanical resistance on the part of the substrate object.
Experiences shows that the above-described method, while it is a well developed technique for products with mechanical properties to which it is suited, is accompanied by a number of problems when applied to particularly fragile substrates or products such as glass.
After the substrate object is inserted between the two plates of the mold, there are two requirements which must be satisfied; the object must be held firmly between the plates, to prevent movement of the object, while at the same time the plastic material must be injected only over a previously delimited zone, without leakage or rough edges.
In the case of a substrate material which is not fragile, proper clamping may be assured by providing metal bosses in the structure of the mold, which bosses are employed to clamp the substrate object. But where the substrate objects are curved glass sheets produced in serial production, wherewith inevitably there are differences in curvature from one glass sheet to another, the use of rigid clamping and barrier elements can cause stresses beyond tolerance, resulting in breakage of the glass. Further, during the insertion of the glass sheet between the plates of the mold, longitudinal slippage may occur between the clamping element and the glass, which can cause undesirable scratching of the glass.
The constituent material of the clamping element which contacts both the glass and the molding material must be compatible with the latter, and in particular the molding material must not adhere to it. Further, said constituent material must have good mechanical resistacne to heat, in order to withstand the injection temperature of the molding material.
While one must not use an excessively rigid clamping element, one must also exclude excessively soft elements. Otherwise there is the risk that when the plastic material is injected, under the effect of the injection pressure, said plastic material will leak out of the region established for it. Thus a toric hermetically sealing seal alone may not be sufficient to achieve the desired result. It needs to be relatively soft in order to accomplish a seal, but as a result of this softness it may be unable to clamp the glass sheet tightly enough to avoid displacement and it may be unable to withstand the pressure of the injected material.